The objectives of this study are: a) to determine the importance of extravascular forces, tethering, and axial strain on in situ vein shape and properties, b) to quantify and correlate anatomical variations in vein structure, composition, mechanical properties and hydrostatic pressures, c) to determine the long term role of pressure in the control of wall composition and properties, and d) to quantify the involvement of selected peripheral veins in the carotid sinus reflex. Shape changes will be studies by measuring differential pressures across isolated perfused segments and by 3 view radiography over a wide range of pressures. Wall composition will be determined histologically and chemically, and together with diameter pressure relations, will be used to calculate wall stress and visco-elastic properties. The long term role of pressure will be studies by surgically creating arterial-venous fistuals in one hindlimb per animal and after 30 days analyzing the properties and composition of the femoral, lateral saphenous and dorsal metatarsal veins. Finally, diameter-pressure relations will be determined for 5 anatomically different vein segments at bilateral carotid sinus pressures of 50, 100, 150 and 200 mmHg.